This application claims the benefit of a Japanese Patent Application No. 2002-164563 filed Jun. 5, 2002, in the Japanese Patent Office, the disclosure of which is hereby incorporated by reference.
1. Field of the Invention
The present invention generally relates to adaptive antenna units, and more particularly to an adaptive antenna unit for a mobile terminal, which adaptively controls an antenna directivity in a direction of a base station which transmits and receives signals with the mobile terminal. The present invention also relates to a mobile terminal which uses such an adaptive antenna unit.
2. Description of the Related Art
Recently, mobile (or wireless) communications are becoming increasingly popular. As a result, transmission techniques, including transmission techniques which use microwave bands, transmission techniques having a large transmission capacity, and transmission techniques capable of suppressing interference, have become very important.
On of such important transmission techniques in the mobile communication is a technique which uses an adaptive antenna unit. The adaptive antenna unit is particularly suited for use in a mobile terminal (or mobile station) for making the mobile communication which requires a large transmission capacity, a high-sensitivity signal reception, reduced size and weight of the terminal, and low power consumption.
FIG. 1 is a functional block diagram for explaining an example of a conventional adaptive antenna unit. The adaptive antenna unit shown in FIG. 1 includes antenna elements 11 through 1n, variable phase circuits 21 through 2n provided in correspondence with the antenna elements 11 through 1n, a phase control circuit 3, a combining circuit (Σ) 4, and a reception circuit 5. Reception signals from the antenna elements 11 through 1n are given phase changes in the corresponding variable phase circuits 21 through 2n, combined in the combining circuit 4, and demodulated in the reception circuit 5.
The phase control circuit 3 determines amounts of phase changes to be given at the variable phase circuits 21 through 2n, using output signals of the variable phase circuits 21 through 2n and an output signal of the combining circuit 4, so that a signal-to-interference-plus-noise ratio (SINR) of the output signal of the combining circuit 4 becomes a maximum. For example, the phase control circuit 3 determines the amounts of phases changes to be given at the variable phase circuits 21 through 2n based on an algorithm using minimum mean square error (MMSE). Hence, the phase control circuit 3 controls the amounts of phase changes of the variable phase circuits 21 through 2n, and forms a directivity.
FIGS. 2 and 3 are diagrams for explaining a directivity formed by an adaptive antenna unit of a base station. As shown in FIG. 2, when a first base station 14-1 is exchanging signals with first and second mobile terminals 14-2 and 14-3, signals transmitted from second and third base stations 14-4 and 14-5, other than the first base station 14-1, become noise with respect to the first base station 14-1.
In this case, as shown in FIG. 3, an adaptive antenna unit of the first base station 14-1 forms a directivity having a large gain with respect to directions of the first and second mobile terminals 14-2 and 14-3, and forms a directivity having a zero gain with respect to directions of the second and third base stations 14-4 and 14-5 which become noise sources.
FIGS. 4 and 5 are diagrams for explaining a directivity formed by an adaptive antenna unit of a mobile terminal. As shown in FIG. 4, when a first mobile terminal 15-1 is exchanging signals with a base station 15-2, signals other than the signal received directly from the base station 15-2, become noise with respect to the first mobile terminal 15-1. The signals which become noise with respect to the first mobile terminal 15-1 include interference input through reflections by buildings and the like, noise input through reflections by remote mountains and the like, and signals transmitted from a second mobile terminal 15-3 other than the first mobile terminal 15-1.
In this case, as shown in FIG. 5, the adaptive antenna unit of the first mobile terminal 15-1 forms a directivity having a large gain with respect to a direction of the base station 15-2 with which the first mobile terminal 15-1 exchanges signals, and forms a directivity having an extremely small gain or a zero gain with respect to a direction of a noise source such as the second mobile terminal 15-3 other than the first mobile terminal 15-1, the interference and the reflections.
Therefore, by forming the directivity which has a large gain with respect to the signal exchanging direction and a having substantially zero gain with respect to directions other than the signal exchanging direction, such as directions of communication equipments which become noise sources, it is possible to suppress the noise and the interference. In addition, it is possible to reduce the transmission power and reduce the power consumption, because the signals are transmitted in only the necessary direction and no signals are transmitted in the unnecessary directions.
For example, a mobile terminal which has the directivity by use of an array antenna is proposed in a Japanese Laid-Open Patent Application No. 11-284424. According to this proposed mobile terminal, the directivity is formed so as not to form a beam with a large gain in a direction towards a human head which has a large attenuation.
The situation of the mobile terminal is different from that of the base station. As shown in FIGS. 4 and 5, the signal exchanging direction required for the communication is only in one direction towards the base station 15-2 which relays the communication. Hence, the directivity of the mobile terminal 15-1 should suppress the noise sources including the transmitting signals from the other mobile terminals 15-3 and the base stations other than the base station 15-2, and the interference and reflections from the mountains and buildings.
Due to the recent progress made in semiconductor technologies related to mobile communications, it is no longer impossible to realize a mobile communication system which uses a microwave to millimeter wave band radio frequencies (RF), carries out a high-quality transmission comparable to those of fixed communication networks, and carries out a high-speed transmission on the order of several hundred MHz or greater. However, in the mobile communication system (or cellular communication system), problems such as increased radio wave propagation loss and difficulty in increasing the cell diameter as the frequency becomes higher, and difficulty in suppressing the effects of spreading delays caused by reflection, scattering and diffraction due to buildings, mountains and the like, become more notable. In addition, because a high-speed transmission is required and it is necessary to increase the power per bit of the high-speed data, there is a problem in that the transmission power becomes considerably large. Therefore, the following objects (A1)-(A3) need to be achieved.                (A1) Reduced inter-cell interference;        (A2) Suppression of delay waves (long delay waves) of long delay times; and        (A3) Reduction of required transmission power.        
The adaptive antenna technology is a promising technology for achieving the above described objects (A1)-(A3). In other words, the adaptive antenna technology can achieve the following effects (B1)-(B3).                (B1) Elimination of interference from other cells;        (B2) Suppression of long delay waves (interference); and        (B3) Reduction of transmission power by an antenna gain amounting to the number of antenna elements.        
Particularly in the case of a down-line from the base station to the mobile terminal, a larger transmission capacity is required than an up-line from the mobile terminal to the base station. For this reason, it is desirable to employ the adaptive antenna technology not only in the base station but also in the mobile terminal. However, when applying the adaptive antenna technology to the mobile terminal, the conditions for the mobile terminal is much more severe than those for the base station in order to realize reduced size and weight, reduced power consumption and reduced cost of the mobile terminal.